1. Field of the Invention
The present invention relates to a positive resist composition used in a manufacturing process of semiconductor, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic processes of other photo-fabrication, and also the invention relates to a pattern-forming process using the same.
2. Background Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light sources and increasing of the numerical aperture (high NA) of projection lens have advanced, and now exposure apparatus of NA 0.84 using an ArF excimer laser having a wavelength of 193 nm as the light source have been developed, which can be expressed by the following equations as generally known:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are the coefficients concerning the process.
As a technique for increasing resolution in optical microscopes, a so-called immersion method of filling a liquid of high refractive index (hereinafter sometimes referred to as “immersion liquid”) between a projection lens and a sample has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following equations in the case of immersion, with λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sin θ with θ as convergence half angle of the beam of light:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same NA, the depth of focus can be made n magnifications by immersion. This is effective for every pattern form, and further, this can be combined with super resolution techniques such as a phase shift method and a deformation lighting method now under discussion.
When ArF excimer laser is used as a light source, it is thought that pure water (refractive index at 193 nm: 1.44) is most promising in view of the safety in handling, and the transmittance and the refractive index at 193 nm.
Concerning immersion exposure techniques, a pattern-forming method on the basis of the process for immersion exposure is disclosed in patent literature 1 (JP-A63-49893, the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”), resist compositions for use in a resist pattern-forming method including an immersion exposure process are disclosed in patent literature 2 (WO 2004/068242A1), and resist protective film-forming materials for immersion exposure process are disclosed in patent literature 3 (WO 2004/074937A1).
To explain the image-forming method of a chemical amplification type resist composition by example, this is an image-forming method of exposing a resist composition to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as a reactive catalyst in the bake after exposure PBE: Post Exposure Bake) to change an alkali-insoluble group to an alkali-soluble group, and removing the exposed area by alkali development.
The resist for ArF excimer laser (193 nm) using the mechanism of chemical amplification is now becoming the main stream, but exposure latitude from exposure to post-heating is not sufficient, so that further improvement is required.
Further, when a chemical amplification resist is applied to immersion exposure techniques for further fine pattern forming, a chemical amplification resist that undergoes no lithographic problem in general dry exposure causes fluctuations of sensitivity by immersion exposure, so that the improvement is required.